Let's say some aliens come to Earth and prank us (apparently they never heard of the prime directive) by taking the earth, and magically moving the two halves (you can pick which two make it most interesting) apart from each other such that the distance between them is the same as the old diameter of the earth. Also, to be nice, they moved the moon into orbit around the sun so we wouldn't get a three body problem. Other satellites are placed into miscellaneous orbits around each part. Also, the two halves are set into rotation into each other so they maintain roughly the same distance. After they have done this, the magic stops, and the aliens leave.

What happens now? I am looking for both short term and long term answers.

$\begingroup$I'm guessing the two halves would collapse into each other in a cataclysm. Are you suggesting a sundering, as you would cut an orange in half and separate it?? Two hemi-spheres? They would just fall together tragically. Or am I not understanding the question.$\endgroup$
– MikeyJun 3 '15 at 0:45

$\begingroup$There was another Question on a "half planet" not to long ago. Much of the response concerning the individual halves (squrting, collapsing) duplicates that.$\endgroup$
– JDługoszJun 3 '15 at 4:52

6 Answers
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Well the first thing that happens is that both halves immediately start collapsing into a sphere smaller by half the volume of the Earth. This would be a cataclysmic event unto itself. Likely nothing would survive for long. There was another question about taking out a huge hunk of the earth, all the same stuff as that only worse.

if the 2 halves where not set to orbiting each other at a decently fast speed, they would immediately begin to move toward each other and merge back together in another huge cataclysm. Either way this is the likely outcome, it just matters how long it takes will be dependent on how fast the 2 are set to orbit each other. Being only the distance of the diameter of the original Earth, I think the mutual orbit would have to be pretty fast, fast enough to cause other problems.

But in every case, everyone and everything stuck on the planets are going to die in short order.

$\begingroup$I said that they were orbiting each other.$\endgroup$
– PyRulezJun 3 '15 at 0:49

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$\begingroup$@PyRulez yes, I know that, at such a close range it doesn't really matter. On top of that, everyone would die long before they collided back together because of the huge tectonic forces turning the hemispheres back to spheres.$\endgroup$
– bowlturnerJun 3 '15 at 0:51

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$\begingroup$@PyRulez They are more than big enough to immediately start the process, with a molten core that is released making it even easier. It might take decades or centuries but the immediate tectonic forces would shake everyone to death quickly.$\endgroup$
– bowlturnerJun 3 '15 at 0:55

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$\begingroup$Not sure why this was downvoted, +1 to counter. The immediate effects would be catastrophic and globally life-ending as the half-spheres partially collapsed and started to reform.$\endgroup$
– Dan SmolinskeJun 3 '15 at 1:07

Well, assuming that the dividing line runs through the Atlantic and Pacific Oceans (let's assume 23 W longitude and 157 E longitude), the oceans of the world will drain in a matter of days. Both the Pacific and Atlantic Oceans (all 4) will drain immediately to a depth of at least 15,000 feet, the Arctic Ocean will drain completely, and the Indian Ocean will drain around Antarctica to the same depth, although that will take a few days longer. Not that the earth has a few weeks to watch the process.

Oh yes, and the atmosphere will also drain off, just like the oceans.

EDIT - And it occurs to me that this needs a bit of expansion. Let's assume that the two halves, once split, are alienly moved apart a few thousand miles in the wink of an eye. Then the entire edge around the cut plane will experience what you might call "horizontal explosive decompression", which will actually become a high wind, as the air farther away from the edge expands/is pulled to the edge and beyond. Once over the edge, the air will fall toward the core, and form a bubble sitting on the cut plane. The edge of the disturbance zone will propagate inwards at the speed of sound, so it will be about 10 hours before it reaches the center of the hemisphere. Assuming the atmosphere is uniformly distributed over the new surface, the average density of the new atmosphere will be 2/3 that of the original. However, the air will not be uniformly distributed, since air will fall off the edge. Frankly, I'm not going to do the calculations to figure out what the new distribution will be, but I suspect that quite a large fraction of the available air will be lost to the old surface. In addition, what air there is will be less dense than a similar amount of air pre-split. The reason is that the surface gravity on the old surface will be less than previously, so the atmosphere will not be as concentrated at the surface. At a guess, the air density will be around 1/3 the present value, or about what you find at the top of Everest. It will take a minimum of a day to reach equilibrium (speed of sound from edge to center of old surface to start redistribution, and more than that for the air to flow to its final location) At the edge, the initial winds will be an appreciable fraction of the speed of sound, since the air is expanding freely into a vacuum, and this will help empty the oceans quickly.

END EDIT

The core of the Earth starts about 1800 miles from the center, so the liquid core has a diameter of ~3600 miles. At the outer limit of the core, the viscosity of the molten iron is roughly 100 poise, or somewhere between water and motor oil. Pressures at this depth are ~300 GPa, or 300 million atmospheres. Viscosity increases with pressure, which increases with depth, so the inner core becomes solid.

With the bisection of the earth, the pressure in the core will drop essentially to zero, since core material can squirt out sideways, and the entire core will become liquid, with an initial temperature in the 4000 to 6000 K range. With the central core liquid, there will be no possibility of the edges maintaining shape, and the earth will immediately begin to flow downwards all around the new flat face. The process will continue until the Earth has reached a sphere with a diameter of about 6400 miles. There will be precious little of the original surface left. What there is will be molten, since the gravitational energy released in the collapse will have nowhere to go except as heat. Well, there will be a certain amount of ejecta normal to the plane of bisection.

The energy delivered to crustal material can be roughly estimated as follows. A mass at the old surface (4000 miles from center) will fall 800 miles (1280 km) to reach the new surface. The new surface will have a gravitational acceleration of 9.8 / 2 ^ 1/3, or about 7.8 m/sec^2. The energy imparted to a 1 kg mass will then be mgh, or just about 10,000 kJ. The specific heat of granite is .8 kJ/kg-K, so the material will heat up about 12,000 degrees. Toss in a correction factor to allow for the decreased gravity at altitude above the new surface, and 9,000 to 10,000 degrees seems reasonable. In other words, the new sphere will be molten where it isn't gaseous.

I've not included the energy required to separate the two halves, and this will mitigate the thermal effects some, but it still ought to be quite a show.

The inner core of the Earth is made of solid iron and nickel. Solid because the pressure is overwhelming; without the pressure, the temperature would make both metals - no, not liquid, gaseous. Take off the pressure, wich is implied in cutting the planet to half, and a monsterous eruption of iron gas takes place; with the core suddenly losing consistence, all the upper layers immediately crash down; the crust of the remaining half is destroyed in the process. The atmosphere quickly divides into a cold layer of oxygen + nitrogen, and a nightmarishly hot layer of iron + nickel gas. At the temperatures involved, iron and nickel burn. And then they cool down, and it rains... liquid or red hot nickel, iron, and their respective oxids. Oceans boil, due to both increased temperature and reduced pressure. And no, it doesn't take years for all this to happen, or even months. It is an extinction level event up to eleven - I doubt even bacteria would survive the process.

The end result is two smaller spheric, dead, sterile planets orbiting each other. Both violently whipped by solar wind, as their disrupted cores would quite certainly be unable to generate a magnetic field.

Of course, since this is aliens with the appropriate technological/magic means to split the Earth in two, they may also have the means to keep the hemispheric shape of each half - perhaps they seal the planetary wound with whatever was that constituted the dome in Stephen King's Under the Dome; or they know the correct spells to keep everything in place. In this case, the point most distant from the edge of the cut is closer to the new gravitation center, and consequently everything falls towards it. Again, extinction-level event.

OK again, the sufficiently technologically advanced - ie, magic - aliens can change the laws of physics in a way that the gravitational center of each semiplanet no longer coincides with its baricenter; somehow it remains in the flat surface of the hemisphere. And, as new difficulties are raised, the aliens counter-attack with newer magic - I mean, sufficiently technologically advanced - tricks. You win; life is preserved in both semi-planets, and the main difference is that London-Sydney is now an interplanetary trip, not merely an international one. If we come to this point, then the question is: on what plane was the Earth cut by half? The Equatorial plane? The Eclyptic? Or along a meridian (which one?) According to that answer, we could have semiplanets where it is either summer in the whole surface, or winter. Which countries and oceans would be cut apart between semiplanets? In the case of countries, how do split countries reorganise? What does a Washington DC-less no-longer-United States do? New capital, new elections for president? Or just chaos? What resources become suddenly scarcer in each half? What happen to migratory species whose migration routes are severed? Do we discover that one of the halves, or both, is no longer a sustainable environment without the other half? And those poor Western corporations, how do they manage to stay afloat without Chinese sweatshops? How do Muslisms fare in a half-world without Mecca (or Catholics without Rome?)

If you put the planet in stasis, cut it in half, separated the two halves by a few thousand kilometers, and then set them spinning around one another and released them from stasis, what would happen is that the entire liquid mantle, super-hot and under immense pressure, would immediately and simultaneously boil and try to flow down towards the former center of the planet (being the new surface of the half-planet and the new low point relative to the center of the half-mass) in the largest lava flow seen since the moon was formed. All of these trillions of tons of molten metal would be rushing down to the center, some of it falling literally thousands of kilometers, and then when it gets to the bottom, it would slam headlong into all the other magma coming up from the other direction.

So trillions of tons of boiling-hot metal are now slamming together at extremely high speeds. This collision at the former center of the planet basically makes each half of the planet into a planetary-scale shaped charge, blasting hypersonic jets of molten nickel and iron into space, directly at the center of the other half. As a small-scale example of what I'm talking about, ever see a couple of waves come together and throw a bit of spray up into the air? Imagine that on an Armageddon scale.

Now, because the two hemi-planets would need to be rotating extremely fast to maintain separation, it's possible that the two jets don't just crash directly into one another. Instead, they probably slam into the trailing side of the opposite half of the planet, sending off another giant spray of molten ex-planetary bits, and possibly even hitting hard enough to blast significant chunks off the other planet-half.

Basically, it's messy. And the resulting halves would almost certainly each be inside the other's Roche limit, meaning that the two halves would tear each other apart due to tidal forces, turning our former planet into a rapidly-rotating disk of ex-planetary debris. Over time, this would probably re-coalesce into a somewhat smaller planet. The rest of the solar system would see a heavy uptick in the volume of meteors for thousands or millions of years due to the Earth fragments sent flying off in the initial split or in subsequent fragmentary collisions. Venus and Luna especially, being in the closest orbits, would be hardest hit. Eventually Jupiter's influence would either send the wandering bits either off into interplanetary space, or corral them into the asteroid belt.

Needless to say, life on Earth ends in spectacular fashion shortly after the stasis is released.

--Edited to add--

I realized shortly after posting that I'm overlooking an important effect. The massive jets formed by the flash-boiling and collapse of the mantle and half-cores would act as a pair of mass-drivers, accelerating much of the remaining mass of the half-planets, including the bulk of the crust, outwards at high speed, possibly even reaching the escape velocity of ex-Earth. This would hasten the formation of a disk of debris, and possibly result in either a new pair of dwarf planets or some rather violent collisions as the remains coalesced or collided with various other bodies in the solar system.

Such a binary system would rotate around the center of mass, in this case that would be the actual center of the two halves.

Collapse:

The earth's core stays solid due to pressure. By splitting the earth, the aliens have released this pressure. The earth's core would spill out as a liquid but would still be attracted to the huge gravitational mass which is the half-sphere. The same would happen with the mantle. Evenually, each half-sphere would collapse in on itself (very slowly, it would take a while), creating two smaller sphere.

Effect on humanity:

The (eventual, it will take a while) collapse of each half-sphere would wreck havoc on the surface, destroying buildings and cities. The binary procession of the two half-spheres would ruin our seasonal calendar. Travel and trade routes would be ruined. Countries would lose international trade. They could still travel between halves, but it would be so expensive that you would not make any profit. Radio, cell phone, etc. would still work, but many families would be separated. The economy would undergo a mass depression. The spilling of the mantle has destroyed the earth's magnetic field. Cosmic rays wreck havoc on each half-sphere surface. These aliens have done a lot more than just pranking us.

Earth has many layers of material, each with a different composition, pressure, temperature. When the pressure is released, these layers of magma, molten metal, and etc. are going to pop.

Mythbusters shows what happens when you release high-pressure, high-heat layered material from a crust that holds it all together. Spoilers: someone gets burned. With the earth, that someone is everyone.

Thank you for your interest in this question.
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